1. Field of the Invention
The present invention relates to a rotor drive controlling unit for reducing a rotation period fluctuation of a rotor, when the rotor is rotationally driven by a motor, and the like; and an image formation apparatus including the rotor drive controlling unit.
2. Description of the Related Art
With reference to FIG. 1, an image formation apparatus, in general, is described. FIG. 1 shows a color image formation apparatus such as a 4-color tandem type color printer. The image formation apparatus includes a controller 5 for controlling the entirety of the image formation apparatus, and photo conductor drums 1a through 1d. A latent image representing an image in black color is formed on the photo conductor drum 1a; a latent image representing an image in cyan color is formed on the photo conductor drum 1b; similarly, a latent image for magenta is formed on 1c; and for yellow on 1d. The image formation apparatus further includes exposing units 2a through 2d for forming the latent images for the respective colors on the corresponding photo conductor drums 1a through 1d. The image formation apparatus further includes motors 6a through 6d for rotating the corresponding photo conductor drums 1a through 1d. A belt 3 is driven by a belt driving motor 4 for conveying an imprinting medium 7, such as paper.
Next, operations of the image formation apparatus shown by FIG. 1 are described. First, the imprinting medium 7 is conveyed to the belt 3 from a feed unit that is not illustrated, delivered to the belt 3, and sequentially conveyed to the photo conductor drums 1a through 1d for each of the colors. At this time, the latent images are formed on the corresponding photo conductor drums 1a through 1d from above by the corresponding exposing units 2a through 2d. Then, toner is transferred to exposed parts of the corresponding photo conductor drums 1a through 1d. The toner is then transferred to the imprinting medium 7 that comes just under each of the photo conductor drums 1a through 1d. In the image formation apparatus as shown in FIG. 1, the photo conductor drums 1a through 1d are driven by corresponding DC brushless motors, and the like. Due to the following reasons (i) and (ii), the formed image tends to have a positioning error in sub scanning directions, namely:
(i) motor rotation period fluctuation due to a torque ripple, and the like; and
(ii) an error caused by a driving force transfer system such as an accumulated gear pitch error, and an eccentricity of a rotating axle.
The positioning error in the configuration shown by FIG. 1 occurs, for example, when planet gears are used between the motors 6a through 6d and the photo conductor drums 1a through 1d, respectively. The positioning error due to the errors occurs not only in the case of the configuration shown in FIG. 1, but also in the case of a revolving system wherein images in two or more colors are formed with one photo conductor, and in the case of a monochrome system with one photo conductor.
The image formation apparatus that is configured as shown in FIG. 1 is capable of delivering a color image at high speed, and accordingly, is widely used. With this configuration, the positioning error between images formed in different colors results in erroneous superposition of the colors, then the so-called color shift occurs, and image quality is notably degraded.
Conventionally, various countermeasures are taken in order to improve the quality of images produced by image formation apparatuses. Concerning the rotation period fluctuation of a DC servomotor, a control system is used, wherein the angular velocity of motor axle rotation is detected and fed back. Further, concerning the error due to the driving force transfer system, a rotary encoder is provided on the axle of the photo conductor drum such that rotation of the motors 6a through 6d is detected and controlled. Furthermore, in a manufacturing stage, the maximum eccentricity position of gears on the same axle as the photo conductor drum axle is detected, and the four photo conductor drums are assembled while adjusting the gearing eccentricity positions of the photo conductor drum axles. In this way, each phase of the rotation period fluctuation due to eccentricity is synchronized, and the color shift is mitigated.
As a method of mitigating the color shift by synchronizing the phases of the rotation period fluctuations, which are periodic, of two or more photo conductor drums, Patent Reference 1 and Patent Reference 2 propose that a reference position be predetermined. At the reference position, the phases of the rotation period fluctuations of the photo conductor drums become the same so that the photo conductor drums can be driven with the phases of the rotation period fluctuations agreeing with each other, and so that imprinting can be carried out at the same position. Further, as described above, the method can be carried out by detecting the maximum eccentricity positions of the gears on the axles of the photo conductor drums, and assembling the photo conductor drums with highly precise axle matching so that the phases can be aligned in order to mitigate the color shift that may occur when superposing two or more colors.
Even if the phases of the rotation period fluctuation are aligned so that the color shift due to the photo conductor drum rotation period fluctuation can be mitigated by the method, amplitudes of the rotation period fluctuations differ with the photo conductor drums. The difference in the amplitudes causes the color shift. That is, even if the phases of the rotation period fluctuations of the photo conductor drums are aligned for reducing a relative amount of the color shift, the color shift due to the difference in the amplitudes of the rotation period fluctuations remains. Accordingly, in order to obtain a high quality image with less color shift, it is necessary to reduce an absolute amount of the amplitude. Here, it is known that a positioning error of a pixel due to the amplitude of the rotation period fluctuation during one rotation of the drum is greater than a positioning error of the pixel due to the amplitude of a rotation period fluctuation of other devices.
In this connection, Patent Reference 3 proposes a method of reducing the amplitude of the rotation period fluctuation, wherein the frequency of the rotation period fluctuation is analyzed, a frequency component for compensation is detected, and control is carried out. However, according to the method of Patent Reference 3, a great number of elements-to-be-detected such as slits of an encoder for detecting the rotation period fluctuation are required; accordingly, the cost of the structure tends to be high.
In an attempt to solve the problem, methods of detecting and controlling only a rotation period fluctuation that affects the image quality are considered. For example, Patent Reference 4 proposes a method wherein a frequency component equivalent to a rotation period fluctuation of a drum axle is calculated by carrying out a frequency analysis of the rotation period fluctuation of the motor axle and by multiplying the frequency component by a gear reduction ratio; then the rotation of a motor is controlled based on a result of the calculation.
Further, Patent Reference 5 proposes a method wherein time between slits of a rotation plate is measured such that a rotation period fluctuation is detected. This method requires a smaller number of slits, or slit intervals, and provides a simpler and more economical solution than a conventional method wherein a rotary encoder detects a rotation period fluctuation by counting the number of slits of a rotation plate passing during a predetermined time.
Patent Reference 6 proposes a method wherein drive-unit control is started based on a specified speed value stored in a storage unit, control is sequentially performed, so that a moving speed fluctuation of an image supporting body is reduced. Further, Patent Reference 7 proposes a method wherein a rewritable storage unit is provided, and even when a rotation fluctuation of a photo conductor occurs due to a temperature change, wear of a reduction gear, etc., change of the speed is detected at a suitable timing and the rotation fluctuation is controlled.
Furthermore, Patent Reference 8 and Patent Reference 9 propose a method wherein an interval between timing pulses that become high level when passing slits is changed so that a reference position to start controlling is detected.
[Patent Reference 1] JP H8-10372
[Patent Reference 2] JPA 2000-137424
[Patent Reference 3] JPA 2002-72816
[Patent Reference 4] JPA 2000-356929
[Patent Reference 5] JPA 2005-312262
[Patent Reference 6] JPA 2000-295882
[Patent Reference 7] JP 3259440
[Patent Reference 8] JPA H6-227062
[Patent Reference 9] JPA H6-234253